Vacuum distillation from moving surfaces and apparatus therefor



VACUUM DI STILLATION FROM MOVING SURFAGES AND APPARATUS THEREFOR Filed Nov. 5, 1939 5 Sheets-Sheet r l fig-2. 4o F1Q-1- 1 2- 26 Kenneth Hickman DISTILLATE OUT Oct. 13, 1942. K. c. D. HICKM AN 2,298,377

VACUUM'DISTILLATION FROM MOVING SURFACES AND APPARATUS THEREFOR Filed Nov. '3, 193s; :s Sheets-Sheet 2 DISTILLATE RESIDUE 4 our DISTILLAND I IN Kenneth C.D.Hi1;kman

Oct. 13, 1942. K. c. D. HICKMAN VACUUM DISTILLATION FROM MOVING SURFACES AND APPARATUS THEREFOR Filed Nov. 3, 1939 3 Sheets-Sheet 3 z w w 2 t 4 0 2 4 9 1 T IIUU H0 W 45m n T 245 wwm 1 w w I 2 a woe/Whoa, Kenneth C. D. Hickman 1 2-20: Fig. 1;

Patented Oct. 13, 1942 VAOUUK DISTILLATION MOVIN SURFACES B Kenneth C. D. Hickman, Rochester, 'N. 1., assignor to Distillation Products, Inc.,' Rochester, N. Y., a corporation of Delaware Application November 3, 1939,Serial No. 302,7 lz'olnlms. (01. 202-42 This invention relates to the art of vaeunm distillation and in particular relates to a method I V of 'high vacuum unobstructed path distillation from moving heated vaporizing surfaces.

In application 99,632, filed September 5, 1936,

now Patent 2,210,927, dated August 13, 1940, I

have described vacuum distillation procedure wherein the substance to be distilled is introduced onto a moving heated surface over which it is caused to pass in a very thin film at a rate substantially greater than would be obtained if the 1 which permits separation and repeated fractionation of a single fraction;

Fig. 4' is a fragmentary section of the ery of a moving distilling surface of the type distilland were caused to flow by ravity. One of the methods ofdistilling in this manner involves introducing the 'distilland onto the approximate center of a rotating disk or plate which is heated to distillation temperature.

This invention has for its object to provide improved distillation apparatus and process of the nature described above. A further object is. to provide high vacuum unobstructed path distillation process and apparatus for distilling from a moving surface whereby fractionation may be accomplished from one moving surface; A further object is to provide a method of distillation whereby distilland can be introduced onto a heated revolving disc, plate, cone, or the like, and a pluralityof fractions separated from the distilland during its passage thereover. A further object is to providemethod and apparatus whereby fractionation from a single moving member is readily accomplished. A further object is to provide improved high vacuum, unobstructed path distillation process and apparatus. Other objects will become apparent from the following descripti n and claims.

These and other objects are accomplished by my invention which includes method and apshown in Figs. 1 and 3 and illustrated in detail the method of removing liquid therefrom;

Fig. 5 is an elevation in section of a modification of a fractionating still of thetype illustrat ed in Fig. 3 being provided with a horizontal vaporizing plate;

Fig. 6 is an elevation in section of asingle ver-.

tical plate still provided with concentric condensing surfaces from which the liquid drains by gravit-y into withdrawal conduits and being provided with baffles; l/1

Fig. '7 is an elevation in section of a multi-plate 'fractionating still provided with condensing surfaces which are'similar to th oseshown in Fig. 6;

and, v

' Fig. 8 is a fragmentary section in elevation of a modified apparatus for removing the final fraction from gutter I of Fig. 5.

Referring to Figs. 1 and 2, numeral l0 designates a squat cylinder which serves as a still casing and which is closed at one end thereof by integral plate I: and at the other end by remov able, but gas tight, plate [4. Numeral l6 designates a conduit of wide admittance integral with 7 plate M which conduit is connected to evacuating pumps (not shown); Numeral l8 designates'a shaft which passes through packed gland 20 integral with plate I! and which is rigidly supported thereby. Numeral 22 designates a circular disc integral with shaft i8 and supported thereparatus whereby vacuum fractional distillation,

i. e.. separation of a plurality of fractions 01' repeated fractionating of a single fraction is accomplished by distillation from one heated moving vaporizing surface. A plurality of such multiple effect moving surfaces can be employed.

In the following description I have set'forth several of the preferred embodiments of winvention.' However, it is to be understood that these are given forthe purpose of illustration and not in limitation thereof. v

In the drawings in which like n nnerals refer to like p r F 1 is a vertical with my invention, permits the separation of a plurality of fractions by one passage of distilland therethroil I Pig. 2 is a section in elevation taken on line Fig. 3 is a vertical section of a single plate still which is'in many respects similar to section in elevation of a 7 single plate centrifugal still which, in accordance annular surfaces disposed opposite to by in the position'shown. Numerals 22', 22" and 22"f designate concentric, annular vaporizing zones on plate 22. Numeral 24 designates a depression in the approximate center of plate 22.

Numeral 2i designates an annular gutter which is formed at the perlphery of plate 22 by turning back the edge thereof. Numeral 28 designates a withdrawal conduit, one end of which terminates inside the gutter 26. Numerals 30, 32 and 34 designate heating coils, the heat from which is applied to areas 22', 225' and 22'", respectively, of plate 22.

Numerals 36, 38 and 40 designate ncentric ncentric zonesl', 22", and 22", respectively, and are of approximately the same areas The circular or annular members 36, 38 and 0 are so constructed and assembled that annularspaces l2 and 44 are .provided therebetween. The upper half of the inside circular edge of member 40 is turned upward to form a gutter It.

The bottom half of the outside edgeof the same member 40 is likewise turned upward to form a gutter 48. The upper half of .theinside clr cular edge of member "is likewise turned up- 60 ward to form a gutter and the lower half of perlphthe outer edge thereof is turned upward to form a gutter 62. The lower half of the outer edge of member 86 is, turned upward to form a gutter 64. -To gutters 48, 62 and 84 are attached withdrawal conduits 86, 66 and 68, respectively. Numeral 62 designates cooling conduits integral with the back portion of concentric annular plates 86, 38 and 48. Numeral 64 designates a conduit which leads into the still and terminates in the recessed portion 24 of plate 22. Numeral 66 designates a pulley integral with shaft I8.

Referring to Fig. 3 numeral 68 designates a conduit which leads into the still and terminates in close proximity to the elevated portion a conduit which connects the bottom of gutter 48 to pump 88. Numeral 92 designates a conduit which runs from pump 88 to the elevated portion of plate 22 between annular zones 22 and 22".

Referring to Fig. 4, numeral I88 designates the end of conduit 28 which terminates inside of gutter 26. This end is curled into the gutter so that it lies near the bottom of gutter 26 and is cut at an angle so that the point of the cut is next to the-bottom of the gutter and points in a direction opposite to thatin which plate 22 rotates.

Referring to Fig. 5, numeral I28 designates an annular gutter into which protrudes the outside edge of plate 22. {The withdrawal conduit I22 is connected to the lower portion of gutter I28. Numerals I24, I26 and I28 designate three concentric cylinders, the lower edges of I24 and I26 being provided with double gutterstumed respectively. Cylinder I28 is provided with inwardly turned gutter I38. Numeral I48 desigdensing ring 228 which cooperate to prevent vapor from passing without at least one contact therewith but which are so located as to permit free passage of permanent gas.

Referring to Fig. 7, which illustrates a still in which two vaporizing surfaces 22 are mounted back to back upon shaft I8, numeral I68 desig- -nates an annular heating element rigidly mounted in the space between the two vaporizing plates. Numeral I62 designates an annular memher, the cross-section of which isin the form i of a truncated cone, the truncated portion of inwardly and outwardly I38, I32, I84 and I36- nates cooling coils integral with each of the concentric cylinders I24, I26 and I28. Numeral I42 designates a withdrawal conduit provided with elevating pump I43 the intake of which is located in gutter I38. Numeral I44 designates a conduit which connects to gutters I32 and I34 and terminates in recess 24 at the center .of plate 22. Numeral I46 designates a conduit' which connects to gutters I36 and I38 and which terminates at the elevated portion of 22 between .annular zones 22' and 22".

' baiiles 226 and 228 which cooperate to prevent vapor from passing through the condensing zone and into the casing of the still without contact therewith. However, the baiiles are so located that ample space for withdrawal of gas is presented. Numerals 238 and 232 designate-annular baffles which are similarly locatedin the space between condensing units 228 and 222. Numerals 234 and 236 designate-baflles located within conwhich extends somewhat into the space between the two vaporizing plates 22. The sides I64 and I66 ofthe annular truncated cone are so. located that undistilled residue thrown from the edges of vaporizing plates 22 comes into contact therewith. The inside edge of the upper half of annular truncated member 162 is provided with an integral flange I68 which extends beyond sides I66 and I64 of the truncated cone member and are turned upwardly so as to form gutters I'll. The outside edge of the lower half of 'the annular' truncated member I62 is provided with turned edges to form gutters I12. A plurality of cooling coils I14 are located inside the truncated cone member and during operation cooling fluid is circulated therethrough.

Numerals I16, I18 and I88 designate concentric cylindrical condensing surfaces the ends of which are cut at an angle so that the walls thereof slope downwardly when the cut ends are mounted substantially parallel 'with the vaporizing surfaces 22. Numerals I82 and I84 designate annular rings integral with the outside surfaces of cylindrical condensing elements I16 and I I8 respectively. The bottom edge of ring I84 is so. situated that liquid dropping therefrom will drop onto the edge of the bottom-upper surface of condensing element I88 in the manner illustrated. The lower portion of ring I82 is similarly located so that liquid dropping therefrom will fall upon the bottom portion of condensing element I18. Numerals I86, I88, and I88 designate a plurality of troughs which are so located within the still casing that liquid dropping from the lower edge of condensing siu'face I82 will fall into trough I86, liquid falling from the lower edge of condensing surface I18 will fall into trough I88 and liquid falling from the lower edge of condensing surface I88 will drop or fall into gutter I98. Numeral I82 designates a conduit connected to pump I84 and to the bottom of gutter I88. 'Numeral I86 designates a conduit which deliversliquid from pump I84 onto the elevated portion of vaporizing surface 22 between vaporizing zones 22' and 22". Numeral I68 designates a conduit which connects to the base of trough I 88 and to pump 288. Numeral 282 designates a conduit which delivers liquid from pump 2 onto the center of vaporizing plate 22. Nu-

meral 284 designates a conduit which connects with the lower portionof trough I86 and serves to withdraw final distillate therefrom.

Numeral 288 designates a rigid bearing support for shaft I8 which shaft is shown as being of large diameter inside the still at I8 but of small diameter where it passes ing 28 at l8". 7

During operation of the apparatus illustrated in Figs.,l and 2, thestill is evacuated by vacuum pumps (not shown) connected to conduits I6. Plate 22 is heated to distillation temperature by electrical heating units 88, 32 and 34, and is rotated by power supplied to pulley 66. Distilland through packed bearis continuously introduced into the recws 24 at the center of plate 22 by way of conduit 64. As

the distilland is introduced onto plate 22, it is fraction is vaporized and condensed on condensing surface-48.

Condensate on surface 36 flows-by gravity into gutter 54 and is withdrawn through conduit 60.

gutter 50 which prevents it from dripping onto surface 36. The condensate on the upper half of '38 thus flows into gutter 50 which directs it onto the lower half of 38 over which it flows into-gutter 52. This fraction is withdrawn therefrom by way of conduit 58. In a similar manner condensate from the upper half of surface 40 collects in gutter 46. is delivered to the lower half thereof and then flows into gutter 48 and is withdrawn through conduit 56. Gases evolved during the distillation or present in the distillation chamber densed upon condensing surface 38 and collects in gutter 52. This. condensate is withdrawn therefrom by pump 88 and delivered by conduit fractionation and removal of lighter constituents Condensate on surface 38 flows downward into 7 88 onto the center of distilling-surfac'e22'. This condensate is thrown over surface 22 by centrifugal force and vapors representing a very pure fraction are condensed on condensing surface 38,

collect in gutter 54 and are withdrawn from the still by way of conduit 82. Lower vapor pressure constituentsunvaporized on surface 22' pass, in sequence over surfaces 22'} and 22" where rethereof takes place. I

It will thus be seen that exceedingly efficient fractionation is accomplished by the apparatus 6f Fig. 3 with removal of a very pure fraction and rejection and refraction of lower vapor pressure constituents which might contain small amounts of the desired component. A plurality of very pure fractions can be prepared by employing a series of units analogous to that illustrated in Fig. 3.

In operating the apparatus illustrated in Fig. 5

distilland is introduced through conduit an arid spread by centrifugal force over area 22". -Undistilled residue is thrown from the periphery of pass around the outside of the condensing surface 43 andthence into vacuum conduit I8 and also pass through openings 44 and 42 between' the condensing segments and then into vacuum conduit I6. Substances which remain undistilled collect in gutter 26 and due to their inertia are throwninto opening I00 and finally flow out of the still through conduit 28. It will be noted that any number of fractions can be separated from a unit of this kind employing a single plate. The number of fractions separated can be varied-by varying the number of vaporizing areas or zones.

In operating the apparatus of Fig. 3 the system plate 22, collects in gutter I20 and is withdrawn through conduit I22. Vapors condense on the outside wall of cylinder I26 and 'on the inside wall of cylinder I28. This condensate is withdrawn from gutters I38 and I38 by conduit I48 and introduced onto the inside edge of distilling surface 22" over which it flows and is redistilled. Undistilled residue therefrom passes onto surface 22". Vapors from zone 22" condense on the outside wall of cylinder I24 and on the inside wall of cylinder I26. This condensate flows into gutters I32 and I34 and is withdrawn therefrom by conduit I44 and introduced onto the center'of vapor-' izing zone 22. Distillation of the final fraction takes place therefrom and it condenses on the inis evacuated and the'vaporizing surface heated H and rotated as described in connection with Fig 1. Distilland is introduced through conduit 80 onto the inside edge of vaporizing portion 22". The distilland then is forced by centrifugal force in the form of a thin film over heated zone 22'.

During such passage distillation of a light fraction takes place. Undistilled residue collects in gutter 26 and is removed from the still by conduit 28. Vapors condense on condensing-ring 43 and collect in gutter 48. This condensate is with- -drawn,by pump 88 and is introduced-by way of conduit 92 onto the inside edge of distilling surface 22' It then flows by centrifugal force over surface 22'. and during its passage thereover side wall of cylinder I24, flows into gutter I38 and ,is withdrawn from the still by pump I43 and conduit I42.

The apparatus of Fig. 5 is substantially -hori-' 'zrnt'al and the condenser is located above the vaporization of the. most volatile constituents 1 takes place. Undistilledportions thereof flow by centrifugal force onto surface 22" where they become mixed with fresh-distilland introduced. p

' thereon and withdrawn through conduit 80. Va-

thereon by way of conduit 30. I It will be realized that the vapors removed from surface 22" contain a substantially higher. concentration of the desired constituent than the vapors removed from 22", and that the undistilled residue from area 22" represents compo:

nents having a lower vapor pressure than the desired distillate which were unavoidably vapor- I therefrom and are then removed'frorn the still Iii vaporizing plate. Therefore condensate flows back onto the different vaporizing areasby gravity. Consequently the apparatus of Fig. 5

has the advantage that pumps 86 and 88 illustrated in Fig. 3 are unnecessary. On the other hand this apparatus has the disadvantage that the gutters at the base of the condensing elements obstruct the passage of vapors to the condensing surfaces to a. certain extent.

In operating the apparatus of rotated and heated to distillation temperatur distilland is introduced onto the" center of the surface by conduit 84. Vapors frg'm zone23' pass into intimate contactwith the inside surface Of 228 and baiiies 234'and 236 arid are condensed pors derived from vaporizing ,zone 22" similarly pass into intimate contactwith the outside surface of condensing cylinder 228,-the inside surface of condensing cylinder 222'and haflles 230 and 232..

This liquid collects in the lower portion of cylinder 222 and is withdrawn from the still by way of conduit. 58. Vapors'from zone 22" condense in the space between condensing cyl nders 222 and 224. This condensate is withdrawn from the tom portion of cylinder 224 by way of conduit 58.

Undistilled residue from the vaporizing plate is withdrawn from the still through conduit-28.

- by conduit :s. Distillate from an. 22" is con Fig..6, plate 22 is The baiiles in theapparatus of Fig. 6 act to effectively condense vapors without the use of 1on8 If desired the condensing.

condensing tubes. units can be made longer to assure contact with vapor, and the baffles eliminated.

In operating the apparatus of Fig. 'l', the still casing is evacuated in the usual manner. Heating element I60 is put into operation in order to heat the distilling plates 22 to distillation temperature. Cooling fluid is circulated through conduits I14 located inside of annular truncated cone I62. Liquidto be distilled is introduced.

side wall of condensing ring I80 and upon the outside wall of condensing ring I18. Liquid condensing on the outside wall of I18 flows by gravity to the base thereof and due to the action of ring I84 drops onto the inside surface of I80. This liquid together with that condensed on the inside surface of I80 fiows into trough I80. This liquid is withdrawn from I80 by pump I94 and delivered through conduit I 86 onto the inside edge of vaporizing zone 22". The liquid passes over zone 22" and vapors therefrom. condense upon the'inside surface of-condensing ring I18 and upon the outtroduced onto the intermediate vaporizing segments instead of on the outer or inner segments.

Undistilled residue is rotation.

,Cundensate from the diflerent condensing zones may be recirculated in a variety. of sequences to improve fractionation or to accomplish a particular fractionating result.

Although I find it useful procedure to heat the different vaporizing zones to different temperatures, this is unnecessary. If desired, the entire vaporizing surface may be at the same temperature but the areas of the difi'erent portions thereof may be varied. It is a characteristic of high vacuum unobstructed path or short path distillationthat substances all distill over any temperature range, but the rate of distillation varies with the area exposed and the molecular weight, as well as with the temperature. If the area is increased the same result is obtained as if- -the temperature were increased, but the area maintained the same or reduced. The area of course controls the time of heating of the distilland, which is also ailected by the speed of The different zones can have equal areas and be heated to successively higher temperatures or they may be heated toequal temperatures with increasing areas or any combination of the two. Whena plurality of such stills are used in series for the purpose of. separating -a plurality of narrow fractions, the temperatures side surface of condensing rims I16; Liquid from the outside surface of I16 flows'to the lowest portion thereof and drops from ring I82 onto the bottom inside surface of condensing ring I18. This liquid along with the liquid condensed on the inside surface of I18 drops into gutter I88. Liquid from gutter I88 is withdrawn by pump 200 and is delivered through conduit 202 onto the center of the vaporizing plates. This liquid passes over zone 22' and vapors thus produced condense upon the inside wall of condensing ring I18. The condensate then fiows to the lowest portion thereof and drops into gutters I88 and is withdrawnfrom the still through-conduit 284.

This apparatus has the advantage that only one evacuating, heating and enclosure system is required for two vaporizing surfaces. It also has the distinct advantage that the heat required for distillation is substantial-lb reduced due to the fact that the heat from the heating unit is utilized in and terminates inside the recess 248. A plurality of holes 244 are provided in the walls ofthe recess. Surrounding these walls is a gutter 248 to which is connected a withdrawal, conduit 248.

During operation condensate flows by gravity Y from I80 into recess 240 by way of conduit .244.

' During-rotation of shaft l8 and plate'22 this 'con- I can toaccompllsh efilcien't fractionation can be densate is thrown by centrifugal force throus holes 244 and into gutter 248 from which it can be withdrawn. a

It will be realized that 'the mode-of recirculavaried greatly. Thus the distilland may in eachstill unit will be appropriate for the fraction which is to be removed therein.

The degree of mixing of vapors of different .fractions in passing to the condensing surfaces will vary with the distance between the condensing and vaporizing surfaces. With short distances the mixing is least. With a 4 foot 5 different forms of apparatus may be employed in combination by connecting them in series.

I have-found my invention to be of particular value for distilling under high vacuum unobstructed path distillation conditions, i. e., at seturation pressures, and/or pressures of residual 8' 01 below 5 mm. such as for instance 1.0-.001

mm. and with condensing and vaporizing surfaces separated by substantially unobstructed space. -When the distance between these surand preferably a distance of about to 6f, the distillation is a form of high vacuum, unobstructed path distillation known as high vacuum, short path distillation. When the distance is" less than the mean free path itis lrnown as molecular distillation. My invention is applicable to all such distillations. In general I prefer to operate with saturation pressures greater than the presssure-of residual gas, though this is not essential, particularly with short distances of separation.

It will be apparent that many modifications can be made in the illustrated apparatus without departing from the spirit or scope of my invention. Numerous details of construction will occur to'a'ny skilled mechanic and many such are'illustrated and described in my application 99,632 referred to above. Reference is made to that application for further details of construction of stills having moving surfaces, their mode 7 v be in: of operation and materials operated upon.

faces isshort such as a distance less than 12'',

tion or fractionating' is improved.

In my co-filed application Serial No. 302,747;

filed November 3, ,1939, I have illustrated a speuum in the space between the plate and the condensing surface.

ciflc embodiment of my invention wherein the description in the above referred to application that surfaces which convey the distilland through the still in a fihn thinner than that obtained by gravity can likewise be used. For instance the distilland may be introduced onto a moving surface of the nature of moving bands or drums, etc., and the intermediate fractions may be returned to the bands or drums in ad vance of the crude distilland. Such surfaces are to be understood as being within the scope of my invention.

What I claim is:

1. The method of vacuum distillation which comprises introducing distilland onto a rotating heated vaporizing surface and separately con densing vapor fractions which are vaporized from difierent areas of the vaporizing surface at successively greater distances from the center .of rotation thereof. a

2. The method of vacuum distillation which comprises introducing distilland onto a rotating circular disc, concentric areas of which are heated to successively higher temperatures in a direction from the center to the periphery thereof and separately condensing the vapors derived from the diflferently heated areas.

3. The method of vacuum distillation which comprises introducing distilland onto a moving vaporizing surface and separately condensing the vapors which are vaporized from different segments of the vaporizing surfaces at successivetlly reater distances from the point of applica- 4. The method of vacuum distillation which comprises distilling from a moving heated surface, separately condensing the vapors in zones at successive distances from the point of application of distilland to the moving heated surface and conveying the distillate from any zone to a point on the moving distilling surface in advance of the point from which it was distilled.

5. The method of vacuum distillation which comprises introducing distilland onto a rotating circular disc all parts of the surface of which are at approximately the same temperature, and separately condensing vapors derived from successive zones of said surface, the area. of the zones progressively increasing from the center totheedgeofthedisc.

6. The method of vacuum distillation which comprises distilling from a moving heated surface, condensing vapors in a plurality of separate fraction upon a condensing surface separated from the moving surface by substantially unobstructed space and returning distilland or distillate to the moving surface whereby purifica- 7. The method of high vacuum, unobstructed path distillation which comprises distilling from 8. The method of high vacuum, unobstructed path distillation which comprises introducing the distilland onto a rotating vaporizing surface which is heated to distillation temperature and which is maintained under a highvacuum, condensing vapors thus produced upon a condensing surface which is separated from the vaporizing surface by substantially unobstructed space, returning the condensed vapors to the heated rotatingv plate at a point different from that at which the distilland was introduced thereon and separately condensing the vapors which are vaporized from this portion of the plate.

9. The method of high vacuum unobstructed path distillation which comprises in combination introducing distilland onto the outer portion of a heated rotating plate which is maintained under a high vacuum, condensing vapors derived from this portion of the plate upon a condensing surface separated from the plate by substantially unobstructed space, introducing at least part of this condensate onto an inner portion of the heated plate which is nearer the axis of rotation j the portion of the plate upon which the distilland was first introduced and separately condensing the vapors derived from this inner section.

10. Vacuum distillation apparatus comprising a rotatable, approximately circular vaporizing surface, means for heating different concentric areas thereof, the center of which is approximately the center of the axis of rotation to different temperatures which progressively increase from the center to the edge of the vaporizing surface, condensing means for separately condensing vapors derived from the different temperature areas and means for maintaining a vacuum in the space between the vaporizing and condensing surfaces. '3

1i. Vacuum distillation apparatus comprising a rotatable circular vaporizing surface, means for heating it to distillation temperature, means for introducing distilland thereon so that during operation it is caused to travel over a portion of the surface near its edge in a thin film by centrifugal force, a condensing surface for condensing vapors derived from this portion of the heated surface, means for returning at least part of the vapors condensed thereon back to the heated surface but to a portion nearer the center thereof than that upon which the distilland was introduced, condensing means for separately condensing the vapors derived from the returned condensate and means for maintaining a vacuum in the space between the vaporizing and condensing 12. Vacuum distillation apparatus comprisin a rotatable vaporizing surface, means for heating it to distillation temperature, means for introducing distilland thereon so that during operation it is caused to travel over a portion of the heated vaporizing surface in a thin film by centrifugal force, a condensing surface for condensing vapors derived from this portion of the heated surface, means for returning at least, part of the vapors condensed thereon back to the heated surface but to a different portion thereof than that 

